Not an easy question.
Hestaby is a very powerful spellcaster and beside Lofwyr the only one we have stats for. (Considering SR4, and Rhonabwy's stats from Prime Runners are a joke, imo. 737kg ... Did they mean Boeing 737?)

However, I think that Lung is probably the most powerful caster of all the great dragons, closely followed by Ryumyo (his erstwhile student), then Schwartzkopf, Hestaby. After those four, it's guesswork, and I'd guess at Rhonabwy for his stunt with the Sea Dragon, or Ghostwalker, being the oldest dragon alive (with possible exception of Lofwyr and Alamaise)

Conjuringwise ... Ghostwalker as the big head honcho (Doll-Maker comes from his knack with spirits as well as his prolific knowledge about the creation of drakes). After that, I have really no idea. Ryumyo probably in the top 5 as well, although something went terribly wrong with his ally spirit. Schwartzkopf somewhere there, too, with all this blurb about golems in his lair. Conjuring and Spellcasting usually goes hand in hand. Hestaby is also known for commanding an army of high force spirits (see SOTA 2073), but I can't guess at the order. Additionally it may depend on the situation

Then there are the physically powerful dragons, Lofwyr, Alamaise, Sirrurg, etc. In metahuman terms, every single one of them can outcast a small army of mages and shamans, but I wouldn't put them in the magical powerhouse category (relative to the more 'magic oriented' dragons, of course).